1. Field of the Invention
The present invention relates to an optical amplifier which collectively amplifies signal light having a plurality of wavelengths in a WDM type optical transmission system.
2. Related Background Art
As optical communications are attaining a greater capacity and higher speed, studies and developments concerning wavelength division multiplex (WDM) transmission systems are being made. One of the most important optical devices in the WDM transmission systems is an optical amplifier which collectively amplifies signal light having a plurality of wavelengths. As such an optical amplifier, an optical fiber amplifier (EDFA: Er-Doped Fiber Amplifier) using an amplifying optical fiber (EDF: Er-Doped Fiber) doped with Er (erbium) element has conventionally been employed.
Such an optical amplifier is provided in each repeater in an optical transmission system. As a consequence, if its gain characteristic (gain deviation or gain slope) is inferior, then, even though a certain wavelength of signal light among a plurality of wavelengths may reach a receiving station with sufficient intensity, the intensity of signal light having the other wavelengths may attenuate such that errors in reception of signal light are caused. Therefore, the optical amplifier is designed and manufactured so as to exhibit a favorable gain characteristic in its signal light wavelength band.
Since the gain characteristic of the optical amplifier also changes depending on temperature, however, even when the optical amplifier is designed and manufactured so as to attain a favorable gain characteristic at a certain temperature, its gain characteristic will deteriorate if the temperature in use changes. In order to eliminate such a problem, there are some known techniques have been known for maintaining the gain characteristic of the optical amplifier.
For example, according to the technique disclosed in Japanese Patent Application Laid-Open No. HEI 4-11794, a temperature control device such as Peltier device is employed so as to control the temperature of the amplifying optical fiber or the like constituting the optical amplifier to keep it constant, thereby maintaining the gain characteristic of the optical amplifier. According to the technique disclosed in Japanese Patent Application Laid-Open No. HEI 5-82873, on the other hand, the amplifying optical fiber constituting the optical amplifier is made shorter, so as to reduce the change in gain characteristic with respect to the change in temperature.